Embodiments of the present invention relate to wireless communication systems and, more particularly, to digital predistortion (DPD) of power amplifier input signals.
Orthogonal Frequency Division Multiplex (OFDM) transmission is widely used in wireless communication systems. With OFDM, multiple tones are passed through an Inverse Fast Fourier Transform (IFFT) to create a time domain signal. The time domain signal is subsequently amplified by a power amplifier and transmitted from a base station (BST) to user equipment (UE) within the wireless network. FIG. 1 shows an exemplary wireless telecommunications network 100 of the prior art. The illustrative telecommunications network includes BST 102. UE 106, and UE 108. BST 102 communicates with internet protocol (IP) network 104 through a wired or wireless telecommunications network. BST 102 subsequently communicates with UEs 106 and 108, which may be cell phones, laptop or tablet computers, internet hot spots, or other wireless network devices.
The power amplifier of BST 102 is typically designed to operate near saturation to achieve maximum efficiency. The transfer function of the power amplifier near saturation, however, is highly nonlinear. In order to compensate for this nonlinearity, input signals of the power amplifier are often predistorted. Referring to FIG. 2A, there is a diagram of digital predistortion (DPD) block 200 and power amplifier 204. A digital input signal VI is initially applied to DPD block 200. DPD block 200 predistorts VI to produce output VP=F(VI) as shown at curve 202. Here, the horizontal and vertical axes of curves 202 and 206 are time and amplitude, respectively. VP is approximately the inverse of the transfer function of power amplifier 204 as shown at curve 206. The resulting output signal VO=F(VI)·G(VP) from power amplifier 204, therefore, is approximately linear as shown at curve 208.
Referring now to FIG. 2B, there is a diagram illustrating the concept of predistortion of power amplifier 204, where the horizontal axis is input power and the vertical axis is output power. At low input and output power, the power amplifier operates in a linear region. As input and output power increase to PIN and POUT 210, respectively, the power amplifier begins to operate in saturation. If uncompensated, operation in saturation causes several problems illustrated by the diagram of FIG. 2C. The desired output characteristic of the power amplifier is curve 220, where the horizontal axis is frequency and the vertical axis is amplitude. However, nonlinear operation in saturation produces adjacent channel interference 224, and a higher adjacent channel power ratio (ACPR) or adjacent channel leakage ratio (ACLR). There are also an in band degraded error vector magnitude (EVM) 222 and third 224 and fifth 226 order intermodulation distortion problems. As input power PIN(DPD) and output power POUT(DPD) of power amplifier 204 increase 212, operation without compensation becomes impractical. With DPD, however, input power is compensated to produce output power 214 in the linear region and reduce problems of FIG. 2C.
Referring now to FIG. 3, there is a Doherty power amplifier of the prior art which is representative of power amplifier 204. The Doherty amplifier includes a carrier amplifier 302 biased to operate in class AB mode and a peaking amplifier 308 biased to operate in class C mode. The carrier amplifier receives RFIN through circuit 300, having an input coupling factor CFI. RFIN is phase shifted by 90° through circuit 306 and applied to peaking amplifier 308. The output of carrier amplifier 302 is phase shifted by 90° through circuit 304 and applied to circuit 310, having an output coupling factor CFO. Circuit 310 combines the output signals from carrier amplifier 302 and peaking amplifier 308 to produce RF output signal RFOUT. The Doherty amplifier is inherently nonlinear due to the two separate amplifiers as well as their respective nonlinear components. Effective digital predistortion requires accurate and detailed modeling of both amplifiers 302 and 308 as well as coupling circuits 300 and 310, phase shift circuits 304 and 306, all components, parasitic coupling of wired interconnect, and any imperfections in power supply networks for transistor drain and gate biasing. Many of the parameters associated with the power amplifier model are dependent on transmit power, operating frequency, semiconductor device temperature and many other factors. Moreover, many of these parameters change over time and must be updated to maintain accurate DPD compensation. However, iteratively updating these parameters is computationally time consuming and may not closely follow rapid changes in the input signal.
While the preceding approaches provide steady improvements in DPD of power amplifiers, the present inventors recognize that still further improvements are possible. Accordingly, the preferred embodiments described below are directed toward improving upon the prior art.